you could use CSS. looks like there is a core.css that SharePoint is using, but you could write your own, or edit the core.css, but editing the core.css is probably not a good idea, it might get overwritten in future versions, and you might mess
up other stuff.

CSS is the way to go, but I'm not a SharePoint expert so I don't know exactly where you would put this CSS file, someone else with more SharePoint knowledge could help there.